Chalcogenopyryliums in the past have been made using either of the two following schemes: ##STR1## wherein R is a variety of substituents and X.sub.1 and X.sub.2 are a chalcogen and are the same or different, and Z is an anion. These reactions have been successful in general in producing relatively pure results. However, I have discovered that a special case exists if X.sub.1 and X.sub.2 are mixed, that is if
X.sub.1 is Se or Te and X.sub.2 is some other chalcogenide, and if Z is a Lewis base. In such a case the end product is unstable, and decomposes back to a scrambled mixture of the starting materials. Thus, at best only a fifty percent pure desired product can be achieved. For pharmaceutical compositions, such a result is unacceptable. This is particularly unfortunate when Z.sup.- is a halide for maximum water-solubility, since halides are reasonable Lewis bases. Impurities will, of course, ruin the effectiveness of the final product, such as when used in cancer therapy.
It is true that compounds of the structure A and B noted above have been taught where X.sub.1 =Se and X.sub.2 =Te. However, in those cases Z was not a Lewis base so that the purity of the product A or B was not jeopardized. Examples of such teachings can be found in U.S. Pat. No. 4,584,258, wherein Z.sup.- is BF.sub.4.sup.- or ClO.sub.4.sup.-.
Thus, prior to this invention there has been a substantial need to find a method for synthesizing relatively pure pyrylium dyes having mixed Se and Te atoms, and a Lewis base anion.